The step joint is a particular type of ultrasonic joint which is useful in joining plastic parts. This type of joint is frequently selected as the exuded melt does not result in a visual defect. Moreover, the exuded melt assists in locating the part and improves the shear resistance of the interface. However, as the nominal wall thickness of the plastic parts to be joined becomes thinner, the wall section required by the typical geometry of the step joint becomes thicker relative to the nominal wall thickness. This localized thick section will create a cosmetic blemish, known as a sink mark, on the surface of the part. This sink mark or depression is created as the hot resin cools after molding.
One prior art solution is to create a style line on the outside of the part to eliminate the extra thickness of plastic which creates the sink mark. In many cases, however, such a style line will not be acceptable because it affects the appearance of the part.
A need therefore exists in the art for a part design which permits a step joint interface which will create a structurally robust joint without creating a sink mark and without creating a style line